I mainly use H20 and Marine Aquatics when my LFS can't supply something.
Both fall foul to the same issue now and again though, and this isn't just limited to the Aquatics industry, but it really gets my back up nonetheless.
Listing something as "in stock" on your website when you don't physically have the item.
I pay for something, then get a call saying "oh actually we don't have any of them" so you ask for a refund and that takes 3-5 days through PayPal. Very frustrating.
It annoys me so much that I've actually delayed the launch of my own business website for 9 months while we ensure that we have a robust stock system that makes selling a product we don't actually have impossible. Hard work, but I don't want to inflict the same fate on my own customers.
Rant over :)